$\begingroup$Let's say that concatenation is allowed except for the trivial case in which you are writing the number that you start with: anyway in this case it is not necessary.$\endgroup$
– mauFeb 22 '16 at 13:35

$\begingroup$Seems good to me. Good job. And here I was trying to write a graph explorer...$\endgroup$
– Diego MartinoiaFeb 22 '16 at 12:24

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$\begingroup$@DarrelHoffman That is not arbitrary at all. By definition we have $n! = (n - 1)! * n$. And if we take $n=1$ we get $1! = 0!$. There is even a (much more involved) generalization to all real numbers (except negative integers).$\endgroup$
– kasperdFeb 22 '16 at 15:18

$\begingroup$@DarrelHoffman You may be interested in math.stackexchange.com/questions/20969/… . The many answers there, including mine at math.stackexchange.com/a/1094926/198422 , suggest a lot of reasons why you'd want $0!$ to equal $1$. I would say that the equation $0! = 1$ is conventionally accepted among the mathematics community: just see the link; there's not really any dissent as to what the "right value" of $0!$ should be.$\endgroup$
– mathmandanFeb 22 '16 at 18:36